1. Field of the Invention
The present invention relates to the field of thermographic relief printing and, more precisely, with the powders used in this procedure.
2. Description of the Prior Art
Thermographics is an established procedure permitting relief printing imitating copper plate printing or stamping from any type of printing process, whether offset or other. The transformation into relief can be achieved, for example, by sprinkling a freshly printed sheet of paper with a powder which has the characteristic of melting under the effect of heat and of forming, after fusion, a film in relief. On the printed parts, the wet ink retains the powder, the excess being continually sucked up and recycled. The printed and powdered sheet then passes through a tunnel oven where it is heated to fuse the powder. At the exit, a blast of cold air cools the sheet and instantaneously sets the viscous film in relief so as to prevent successive sheets from sticking together. Transparent powders can be used, with shiny or matt finishes to thereby preserve the colors of the original printing. If desired, pigmented powders can be used to create a relief corresponding to their pigment.
The granule size of the powder used determines the thickness of the film in relief. The thicker the powder used, the greater the relief. This type of relief printing is used for a variety of different applications such as: commercial work (visiting cards, business cards, letterheads, envelopes, invitation cards, advertising material), labelling on rolls, microprocessing (computer listings) etc.
Over the last eighty years the described procedure has evolved considerably. In the beginning, the powdering of the printed sheets was carried out by hand, then progressively the first machines for transforming into relief became automatic. These machines were still very bulky and were generally reserved for printers specialized in this technique. In the last twenty years the appearance of a new generation of very compact and rapid automatic machines, for use by a large clientel of traditional printers, has meant real industrial development in this procedure.
Unfortunately, these compact machines have also had the length of their tunnel ovens decreased. The amount of time taken for printed matter to pass through these tunnel ovens where the transformation into relief is carried out is, depending on the length of these ovens and the production rate, extremely short, between a half and three seconds. Parallel to this development, offset printing presses to which thermographic machines can be linked automatically, have had their production rates increased by between 1 and 5 times and some of them, specialized in the printing of envelopes, reach a production of 60,000 per hour.
The thermographic powders in use at present throughout the world are mainly of American and English origin, with the exception of a German product which has recently come on the market. The general characteristics of the commercially available powders are more or less the same and only one type of product is proposed to the user by each manufacturer to cover different printing needs and end uses.
Obliged to use these products and in order to bridge the gap created by the performance of the machines and the lack of progress in powders, the equipment designers have been obliged to use very short tunnel ovens which have been provided with increased heating potentials. The temperature reached in the center of these tunnel ovens is typically between 450.degree. and 600.degree. C.
Serious inadequacies and major inconveniences result, of which the principal are the following:
1. The mediocre quality of the film, with craters in the solid parts and a `hammered` surface appearance like an orange skin.
2. A yellowing and partial destruction of the supporting fiber which has been violently dehydrated and subjected to considerable thermal shock.
3. The impossibility of machines equipped with compact ovens of dealing with large size or heavy-weight card as well as more light-weight printed matter which does not stand up to a violent thermal shock.
4. A considerable shrinking in the size of the printed matter resulting from sudden dehydration and partial alteration of the fiber. This uneven shrinking makes it practically impossible to print material intended for collecting into sheaves. This same fault, among others, occurs when printing on sheets or card when flat which are then folded into box shape. The folding and remolding of the form become very difficult.
5. High energy costs making the production cost of the transforming into relief procedure too expensive. In air conditioned premises, the supplementary energy required to compensate for the rise in temperature due to the release of hot air from the tunnel ovens doubles the total energy requirement when transforming printed matter into relief.
6. The power required is too great making it impossible for many small print works to install these machines because of insufficient power being available.
7. Immediate thermal resistance of the film is insufficient to allow it to pass through a modern-day photocopier.
8. A high risk of the printed material catching fire as a result of the oven over-heating.
9. Very poor appearance of relief printed matter especially of that carried out on lightweight supports or on supports easily affected by heat.
10. The impossibility of designing more compact machines with the aim of equipping small printworks with little available space.
The totality of these major inconveniences limits the full development of the prior art procedure which otherwise would have considerable potential were it not for the previously described limitations.
From a study of previous patents carried out at our request by the European Patent Office in The Hague (Holland) in relation to thermographic powders, it is clear that none of the documents referred to teaches the present inventive method. None of the references located teaches a method of thermographic relief printing using a thermographic powder which accelerates the formation of a film on the powdered surface, while at the same time preserving and improving the mechanical or thermal properties of the film.
For reference, these documents can be defined as follows:
U.S. Pat. No. 1,966,907, July 17, 1934 deals with an ink intended to give good flexibility and adherence to thermographic film.
U.S. Pat. No. 2,272,706, Mar. 26, 1938 concerns products giving a shiny surface, good appearance which does not peel and which also has good flexibility.
U.S. Pat. No. 2,226,867, Aug. 11, 1939 deals with the production of a matt powder.
U.S. Pat. No. 2,288,860, June 4, 1940 shows powders for flocking the thickness of which can be increased and controlled.
U.S. Pat. No. 2,317,372, Dec. 28, 1940 deals with a high-temperature ink.
U.S. Pat. No. 2,391,705, Aug. 10, 1942 concerns luminescent powders.
U.S. Pat. No. 3,083,116, Mar. 26, 1963 concerns colored powders.
U.S. Pat. No. 3,440,076, Apr. 22, 1969 concerns inks and powders enabling a relief film to be obtained which is hard and resistant to printing on both sides.
U.S. Pat. No. 3,432,328, Mar. 11, 1969 deals with a process based on a resinous ink allowing for relief printing to be obtained from a stencil.
U.S. Pat. No. 4,044,176, Aug. 23, 1977 is concerned with expansible microcapsules.
British patent No. 713073, Feb. 13, 1951 deals with fluorescent products.
British patent No. 741051, July 16, 1953, concerns a procedure for relief printing where the powder is bound by the action of a liquid product.
British patent No. 881243, Feb. 15, 1960, concerns a presentation of the powder in the form of microspheric grains.
British patent No. 905416, May 27, 1960, deals with thermographic powders with metallic pigmentation.
German patent No. 144744, Sept. 12, 1901, relates to powders with an asphalt base for special printing.
German patent No. 576389, May 10, 1933 deals with powders intended to produce a high quality relief film.
German patent No. 804215, July 8, 1949, deals with powders creating decorative effects.
German patent No 1100654, June 24, 1959, concerns the improvement of the surface state of the film.
French patent No. 449451, Oct. 15, 1912 concerns a procedure for obtaining relief printing from powder and steam.
French patent No. 594642, Mar. 6, 1925 relates to a thermographic relief printing procedure using any powder.
French patent No. 813976, Feb. 14, 1936, deals with powder based on products available at the time, without precise specifications or thermographic machine.
European patent No. 0048478, Sept. 23, 1980, shows a powder making it possible to modify the state of the surface and rendering it sticky. A powder is used on photographs or photochemical products.
Japanese patent No. 5537341, Sept. 8, 1978, deals with transparent microspheric powders, which are infusible, for decorating an inked surface.
Japanese patent No. 585285 A, July 3, 1981, concerns a powder which is comestible.
Japanese patent No. 59142680 A, Feb. 5, 1983, deals with a powder in the form of a toner to replace printing ink.
The present invention has as an object to remedy the foregoing deficiencies by facilitating the production of a range of powder products formulated in relation to the end product and the material at the disposition of the printer.
Another object of the invention consists in decreasing by a significant amount the treatment time of the printed material in a ratio of 1 to 3 depending on the type of support, the purpose of the printed material, the subtlety of the motifs, and the nature and the lengths of the ovens used, in such a way as to accelerate proportionally the production of the machines.
Another object of the invention is, while maintaining the production, to decrease considerably the temperature of the tunnel ovens.
Another object of the invention relates to the improvement in energy costs resulting from the use of these powders. To give an example, the transformation into relief of a printed product carried out on a card 320 grams per square meter in weight and of a width of 1200 millimeters, requires a tunnel oven of about 150 Kilowatts. A reduction in its power of 50% means a considerable energy saving. This saving is doubled in the case of air conditioned premises.
Another object of the invention is to diversify the thermal characteristics of the film and in particular to produce products resistant to a slight increase in temperature, to allow the printed matter, for example, to pass through a photocopier set at 150.degree. C.
Another object of the invention is to improve at the same time the physical qualities of the relief film and its thermal qualities.
Another object of the invention is to avoid the thermal degradation of a fragile support either by lowering the maximum temperature of the ovens or by reducing the treatment time.
Another object of the invention is to limit the abrupt dehydration of their fibers of the paper and their partial thermal degradation, in a manner so as to avoid, limit or reduce, a reduction in the size of the support after treatment.
Another object of the invention concerns the improvement of the tension and the quality of the relief film particularly on solid surfaces carried out on light or heavy-weight supports. This improvement brought to the products also results in an important increase in the production of the machines.
Another object of the invention is to decrease the risk of the support catching fire.
Another object of the invention is to permit, thanks to these powders, the construction of more compact machines than those in use at present.
Another object of the invention is on the one hand, to decrease the rise in room temperature due to the present overheating of ovens, particularly in the case of relatively small workshops where working conditions are sometimes hardly bearable and consequently to improve the functioning of the machines.
Another object of the invention is to substantially lower the price of thermographic powders depending on their use.
Another object of the invention is to bring about, by the totality of the improvements achieved, a significant lowering of the product ion cost for relief printing while maintaining or improving its quality.
Another object of the invention is the obtaining of powders of which the grains are preferably microspheric in form. As will be explained, the grains are preferably of different dimensions, in a specific ratio, to obtain the highest filling rate (packing density) possible of the powdered surface. This approach meets a certain number of criteria which can be collectively defined as follows:
a) Film formation accelerated in comparison to that obtained from the same product when crushed. Indeed, an in-depth study of the transformation, by stages, of a crushed powdered surface into a filmogenous surface revealed under a thermal microscope, that, in the first instance, when the support and the film of powder reach the fusion temperature of the latter, the grains separate from each other and curl up on themselves to form a sort of multitude of more or less perfect microspheres. These grains are actually reacting to a well-known physical phenomenon which shows that any molecular grouping free from all outside forces naturally collects itself into its smallest volume and therefore into a spherical form. When the viscosity of the product and its surface tension are sufficiently low, the spreading out and the formation of a raised relief film takes place. This physical phenomenon does not help in the rapid obtaining of a regular and homogeneous relief film. In effect the grains obtained by crushing have jagged, irregular and anarchic shapes which, before the formation of a relief film, during the transformation stage, give an agglomerate of imperfect microspheres of different diameters which, in a later stage of film formation, have difficulty in joining together and in forming a regular film, without craters. This defect obliges the operator to heat the tunnel oven excessively to abnormally lower the viscosity and to attempt to level the film or to slow down the production.
b) Much improved quality of relief film, particularly on level surfaces.
c) Improvement in the definition of the peripheries of the relief image.
Another object of the invention concerns the method for instant cooling of the powder grains as they come out of the pulverizer, so as to profit from the `memory` of the product to artificially lower the fusion point and in this way the formation time of the relief film. This mode of operating permits, depending on the product, a saving of 0 to 20% on the reheating time of the product and the support. This savings in energy can be added to that produced within the framework of the invention. Also, the manufacture of the microspheres lends itself to the obtaining of grains which set instantly.
It is known that a certain number of properties, including the thermal properties of polymers and in particular of polyamide resins, depend on the degree of crystallinity of the resin. Now the degree of crystallinity is affected by the thermal history of the resin and in particular the manner in which the latter is cooled while it is being formed. Rapid cooling lowers the crystallinity and therefore the fusion point which, by applying this principle of physics to thermographics, makes it possible for the capacity of the powders used to be considerably improved.
Another object of the invention consists in changing the powder distribution hoppers on thermographic machines and replacing them by hoppers allowing the powdering order of the grains to be selected in relation to their decreasing diameter. The improved hopper arrangement also allows sorting and automatic collecting of the powders by compartment.
Another object of the invention is to produce powders which are compatible with each other to enable the user to obtain the best compromise depending on the eventual use of the printed material.
Another object of the invention concerns the thermal properties given to the products to allow them to achieve a rapid recrystallization of the film to prevent the sheets from sticking together, while at the same time decreasing the length of the cooling conveyor.